In today's computer networks, sharing data among devices has become desirable if not essential. Not only does the shared data need to be replicated on each device, but the set of replicated data must be synchronized so that changes made to one replica are reflected in all the others. Synchronization enables many users to work with their own local copy of shared data but have the shared data updated as if they were working on a single, centralized database. For shared data applications where users are geographically widely distributed, replication and synchronization are often the most efficient methods for effectively utilizing shared data.
In addition to desktop computers, workstations, and servers, modern computing environments often include lightweight handheld computing devices that fit into a pocket, purse, or day planner. Modern computing environments range from private networks to the Internet. Although a wide range of application programs can be executed on handheld computers, shared data applications are particularly popular and well suited for these devices. Shared data applications include, among many others, electronic calendars and task lists, electronic mail organizers, and electronic address books. A device running one or more of these applications stores electronic data that is or can be replicated and shared with other devices. It is desirable, if not essential, then to at least periodically synchronize data stored on each device. For example, many calendaring applications allow devices to share data or records concerning each user's appointments. It is important that when a new appointment is added or an existing appointment is modified on one device that the addition or change is reflected on all devices providing calendaring.
Synchronization schemes typically fall under one of two generic categories—“user-initiated” synchronization and “push” synchronization. User-initiated synchronization is commonly employed in “wired” computer networks where computing devices such as server, desktop computers, laptop computers, and personal digital assistants (PDA's) are at least periodically physically interconnected. As its label suggests, this scheme requires a user to commence the synchronization process by, for example, initiating synchronization software. The software analyzes and compares electronic records found on two or more devices and updates each device with new or modified records found on the other devices.
Push synchronization is commonly employed in “wireless” computer networks where the computing devices are able to transmit and receive electronic data via radio frequency, infrared, or any other technology not requiring a physical connection such as a cable between the devices. Typically, for wireless networks, connection speeds between devices are relatively slow and less reliable in comparison to wired networks. This scheme requires no user intervention. Synchronization software monitoring electronic records on one device, typically a server, detects modifications or additions to the records on that device. Upon detection, the synchronization software takes the modified or added records and updates the records located on a remote device such as a pager, cellular telephone, or other wireless device. Push synchronization requires minimal communication between devices, and as a consequence, works well for slower and less reliable networks such as wireless networks.
Where a given computing environment employs one of the above schemes, synchronization is a relatively simple process. However, in modern computing environments, wired and wireless network technologies are often combined making it desirable to simultaneously employ both synchronization schemes. Unfortunately, combining a conventional user-initiated synchronization scheme with a conventional push synchronization scheme inevitably results in unwanted data duplication and/or data loss. Employing wireless networks increases the risk of “trapped” data. Because of the relatively slow connections, sudden unplanned network downtime, and the possibility of the remote device being disconnected from the wireless network connection at any time, it is possible that a pushed record can become “trapped.” While the pushed record is trapped, the device could be synchronized conventionally with the server over a wired connection. When wireless connection is later reestablished the pushed and trapped record is then duplicated.